The agricultural industry uses a variety of fertilizers to apply macronutrients to crop plants, either by application to the soil or application to plant leaves. Nitrogen, phosphorus, potassium, calcium, magnesium, and sulfur are six macronutrients commonly applied to agricultural crops or soil. Nitrogen is commonly applied in the form of urea or as an ammonium salt such as ammonium phosphate.
Urea, by some estimates, constitutes forty-six percent of the worldwide consumption of nitrogen in agriculture and is the most widely used nitrogen fertilizer. But after application to soil the urea compound is susceptible to hydrolysis, which converts the urea to gaseous ammonia and carbon dioxide. The reaction is catalysed by the enzyme urease, which is produced by some bacteria and fungi and can be present in soil. The gaseous ammonia will volatilize to the atmosphere resulting in substantial loss of nitrogen from the total amount applied as urea fertilizer to a field.
To prevent the hydrolysis reaction of urea in a urea-based fertilizer after application, a urease inhibitor may be included in or added to the urea-based fertilizer. The urease inhibitor can prevent conversion of the urea by urease to gaseous ammonia, preventing the loss of nitrogen from the fertilizer to the atmosphere. Preventing conversion of the urea to ammonia will increase the amount of urea (and, necessarily, nitrogen) from the fertilizer that remains in the soil for absorption by a crop plant, making the urea available to plants in the soil for an extended time period. Increasing the amount of time that the urea is available to the plant increases the effectiveness of the fertilizer, which improves crop yield and quality.
Among effective urease inhibitors are the thiophosphoric triamide compounds disclosed in the U.S. Pat. No. 4,530,714 (incorporated herein by reference), including alkyl thiophosphoric triamide compounds such as N-alkyl thiophosphoric triamides. The compound N-(n-butyl)thiophosphoric triamide (NBPT) is the most common species of thiophosphoric triamide compounds currently used in commercial agriculture.
Thiophosphoric triamide compounds, including N-(n-butyl)thiophosphoric triamide, can be in the form of a solid, waxy material that decomposes by the action of moisture and elevated temperature. These materials are known to be difficult to process, such as to incorporate the material into a derivative composition such as a urea-based fertilizer. Desirably, to facilitate processing, the thiophosphoric triamide material can be dissolved in a solvent to form a solution that contains the dissolved thiophosphoric triamide. The solution should meet basic practical requirements including: high solubility and stability of the thiophosphoric triamide compound in the solution; resistance of the solution to crystallization at low temperature; suitably low viscosity of a solution for processing; low toxicity; low volatility; low flammability; minimum content of water; and preferably low cost.
Examples of solvents that have been identified as useful for forming a solution of solvent and urease inhibitor (e.g., NBPT) are described in patent documents that include U.S. Pat. Nos. 5,698,003; 8,163,058; 8,617,425; U.S. Patent Application Publication Number 2013/0134806; and U.S. Patent Application Publication Number 2014/0090432; the entireties of these documents being incorporated herein by reference.
Separately or in combination a urease inhibitor, a “nitrification inhibitor” can be added to or applied with a nitrogen-based fertilizer to prevent denitrification loss. An example of a nitrification inhibitor is dicyandiamide (DCD). When applied with a nitrogen-based fertilizer, DCD can help prevent the loss of nitrogen through denitrification and leaching. But physical properties of DCD create challenges to its use by application to crops or soil. Dicyandiamide in a solid form exhibits a very low solubility in water (about 41 grams per liter), making it difficult to directly incorporate into an aqueous end use fertilizer composition. One method of using DCD is in a crystalline form, which can be added directly to an aqueous-based nitrogen fertilizer. However, the low water solubility of DCD makes this technique difficult. Alternately, to bypass a step of soluhilizing DCD, DCD can be added to molten urea and granulated. See, e.g., U.S. Pat. No. 5,352,265. This eliminates the need to dissolve the DCD in a solvent.
The agriculture and agriculture chemical industries continue to search for still more options for solvents useful for dissolving and processing urease inhibitors such as NBPT, and nitrification inhibitors such as DCD, into commercial fertilizer materials and other products.